I still don't understand how agnostics, by this definition, even think. Of course people can just not believe in a God, it's only foolish of you to try and disprove one. And since Atheists just lack a belief in a God, Agnostics sound like much of the same thing.

Gnostic Theist: There is a God, and I am certain of it.
Agnostic Theist: There's no definite proof, but I lean towards a God.
Agnostic Atheist: There's no definite proof, but I could consider it an extremely vague possibility.
Gnostic Atheist: There is no God, I am certain of it.

Sometimes, the definitions can get a little bit muddled. I'm pretty certain there isn't a God (I abide by Bertrand Russel's teapot model in this case), so you could almost call me a gnostic atheist. However I don't like to claim that us mere humans can really think they fully understand the universe, so in technicality I'm an agnostic atheist. One shouldn't close their mind to considering the supposedly impossible anyway.